Air preheater control



June 8, 1943.

R. S. COOPER AIR PREHEATER CONTROL Eiled May 27, 1941 Nu um J q W W a i g 2 v w v F d a n H. a Y a Han 1 i 5 3 ma Q l mm a mu m$h$wkk SQ SW QREQRQEN \kkfik .KQKQE kw$ Patented June 8, 1943 Roland S. Cooper, River Forest, 111;, assignor to The Air Preheater Corporation, New' York,

Application May 27, 1941, Serial No. 395,352

1 Claim. (Cl. 257 2) The present invention relates to improvements inregenerative air preheaters and particularly to minimizing corrosion of the heat transfer plates in a regenerative preheater of the rotary type. a

When air preheaters are operated under such conditions that the temperature of the metallic heat transfer plates is so low as to be at or below the dew pointof the gases entering the preheater, moisture is condensed from the gases and causes corrosion of the plates. The present inventionvcontemplates minimizing such corrosion by controlling the amount of air passing through the preheater with respect to the amount of heating gases, so the preheater operates under such conditions that the mean temperature of the heat transfer plates is maintained above the dew point of the heating gases. A damper controlled air by-pass around the preheater is provided so more or less air may be by-passed to maintain the plate temperature above thedew point of the heating; gases. This control is based upon the fact that the mean temperature of the heat transfer plate is a definite function of half the total of the temperature existing in the air inlet duct of the preheater and in the gas outlet duct thereof. Accordingly, by providing mechanism for measuring these two temperatures and adding them together the by-pass damper may be automatically adjusted to by-pass air when required so as to maintain a predetermined plate temperature.

In the drawing the single figure is a schematic view of an air preheater installation embodying the present invention.

In the drawing the numeral designates a rotary regenerative air preheater. Air to be preheated is supplied to the preheater by the fan ll through the inlet duct I2 and carried away from the preheater to the point of use through the outlet duct I3. Hot gases such as those discharged from aboiler enter the preheater through the gas inlet duct l4 and pass to the stack or other point of discharge through the gas outlet duct i5. The preheater is of the well known Ljungstrom type, such as is shown in Ljungstrom Patent 1,522,825, issued January 13, 1925, having a face may be .controlledfor: the purposes mentioned above, a by-passduct I16 extendsofrom the air inlet duct I2 to the. air outlet duct,,| 3, and has therein a damper; I]. If desired, dampers [8 may also be provided in the inlet duct 12 or in the outlet duct l3 ahead of the by-pass and arranged so as torestrict flow through the air preheater and" thus divert a, greater quantity through the by-passduct i6. 2 1 Y "I'he dampers "and I8 areconnectedito be actuated by an air operated damper motor diagrammatically represented as consisting tof a piston cylinder 22 which is stationary and a movable yoke 24 which i ,connected .to the piston of the motor and alsoito the dampers ll, 18.

. The temperature of the cool air entering the air preheater is measured by a thermostat 20 and thetemperature of the gas leaving the preheater after having been partiallyocooled'by imparting heat to the air is measured by the thermostat 2|. These'thermostats are. the primary control elements and in accordance with theinvention the by-pass damper i1, is ..to be operated in response to variations from a predetermined total temperature of the entering cool air and outgoing cooled gas so as tomaintainthe heat transfer plates at ajtemperature high enough to prevent condensation of moisturetherein from the heating gases. :Any suitable apparatus Whether vmechanical, electrical or otherwise Aactuated which in its operation would act-to add: together thettemperatures' in the air inlet duct l2 and-gas outlet duct l5 and cause they damper l? to open or close in response to fluctuations from a predetermined total would be suitable for carrying out the present invention.

For the purpose of providing a complete description of the invention one form of such apparatus, which is the "Hagan pneumatic control apparatus, will be described hereinafter although it is to be understood that the invention is not to ;be considered as limited to employment of this apparatus. The thermostats 20 and 21 which are the primary control elements, are associated with other control elements actuated by compressed air and adapted to add the totals of the temperatures measured by these thermostats and eventually cause movement of the dampers I! when a predetermined total temperature is departed from. Compressed air from a suitable source at constant pressureflows through a con duit 30 having an air filter 3| and pressure reducing valve 32 therein to a pilot valve 32A. The pilot valve 33A is connected to the movable element 2! of the thermostat 20 on the air side so that as temperature changes occur in duct l2 the pilot valve 33A is opened more or less thereby varying the air pressure communicated through a conduit 34 to a diaphragm 36 of a Hagan totalizer. In like manner the movable element of the thermostat 2| on the gas side controls the air pressure communicated via a conduit 35 to act on another diaphragm 31 of the totalizer in the same sense that the air pressure in conduit 34 acts on the diaphragm 36. The diaphragms 36 and 31 are interconnected and accordingly the forces acting on the two of them are added together. The diaphragms are connected to and actuate a pilot valve 33T to regulate the air pressure in a conduit 38 in proportion to fluctuations in the total of the temperatures existing at the air inlet and gas outlet of the preheater.

The air pressure created in the conduit 38 to reflect the total of the temperatures in the air inlet l2 and gas outlet I is compared with a standard representing the predetermined temperature at which it is desired to maintain the heat transfer surface of the preheater. .A handset regulating or pilot valve 33M may .be set so that the pressure of air in the conduit 40 constitutes a standard representing the predetermined temperature at which it is desired to maintain the heat transfer surface. The air pressure in conduit 38 representing the actual existing total temperatures of the air entering and the gas leaving the preheater acts'on one side of a diaphragm 4| and is opposed by thestandard pressure in conduit 40 representing the temperature at which it is desired to maintain the heat transferring surface. The diaphragm 4| is connected to' the pilot valve 330 which therefore responds to create a pressure variation in a line 42 in accordance with departures of the actual existing temperature total from a predetermined total temperature.

The pressure in line- 42 acts against a spring on a diaphragm 43 connected to a valve 44 to admit air from a'high pressure line 45 through pipes 46 or 41 to one side or the other of a piston in the cylinder 22 of the air motor that operates the damper ll. When thetotal of the temperatures existing in the air inletduct and gas outlet duct l5 falls belowthe predetermined temperature total the control apparatus produces a pressure in line 42 which causes the Valve 44 to be positioned so that air from the pressure line 45 passes through the pipe 46 to the upperlend of the piston cylinder 22 thereby moving yoke 23 downwardly causing operation of the air motor in a direction to partially open the damper I1. Consequently, part of the cool air previously passing through the preheater flows through the by-pass duct [6. The spring resisting the diaphragm 43 is so arranged as to tend to return valve 44 to a neutral position so as to stop the air motor when the damper I! has opened proportionally to the pressure variation in line 42 caused by decrease in the temperature total. The amount of air passing through the preheater H] from the inlet duct [2 is thus reduced and therefore absorbs less heat from the heat exchange plates. The temperature of these plates is thus maintained at a high enough value to prevent condensation. In other words when the temperature of entering air falls so low or its relation with respect to the temperature of the outlet gases is such that the air would cool the heat transfer plates to a temperature such that on entering the gas side of the preheater condensation will take place, part of the cool air is iby-passed and the plates retain apart of the heat imparted to them by the gases which previously they had yielded to the larger volume of air. When the total temperature measured by the thermostats 20 and 2| exceeds a predetermined value, the resultant action of the control system lowers the pressure in the conduit 42 and the spring associated with diaphragm 43 causes the valve 44 to be moved to a position in which air from pressure line is supplied through the pipe 41 to the lower end of the piston cylinder 22 thereby producing movement of the damper H in the closing direction.

What I claim is:

In a 'regenerativeair preheater having inlet and outlet ducts for a heating gas, similar ducts for air to be heated, and a rotor carrying heat transfer members for absorbing heat from the gases and imparting it to the air; a conduit leading from the air inlet duct to the air outlet ductand so arranged as to form an air by-pass around the preheater; damper means operable to regulate the amount of air flowing through said bypass conduit; means for measuring the air temperature in the inlet duct therefor and the gas temperature in the gas outlet duct; means for obtaining the total -of said temperatures; and

means associated with said totalizing means and responsive to fluctuation of said temperature total from a predetermined total for operating said damper.

- ROLAND S. COOPER. 

